The present invention relates to a process for producing styrene. More particularly, this invention relates to a process for producing styrene from ethylbenzene by the oxidative dehydrogenation method wherein the selectivity of an oxidation catalyst in hydrogen oxidation is prevented from lowering.
Styrene is an important compound as a starting material for polystyrene, synthetic rubbers, ABS resins, unsaturated polyester resins, etc.
Processes for producing styrene by the dehydrogenation reaction of ethylbenzene are known, as described in many documents so far. For example, a process employing an iron-potassium dehydrogenation catalyst is in industrial use.
However, since dehydrogenation reactions generally are considerably influenced by a reaction equilibrium, a high conversion cannot be obtained also in the case of ethylbenzene. Furthermore, when the dehydrogenation reaction of ethylbenzene is conducted in a heat-insulated reactor, the reaction temperature decreases with progress of the reaction because the dehydrogenation reaction is endothermic. This temperature decrease makes it more difficult to obtain a high conversion of ethylbenzene.
The so-called oxidative dehydrogenation method has hence been proposed in which an oxidation catalyst is used together with a dehydrogenation catalyst in the reaction process mainly for the purposes of (1) xe2x80x9cshifting a reaction equilibriumxe2x80x9d and (2) xe2x80x9ccompensating for the decrease in reaction temperaturexe2x80x9d.
For example, Unexamined Published Japanese Patent Application No. sho. 60-130531 describes a method which comprises bringing a hydrocarbon susceptible to dehydrogenation into contact with a dehydrogenation catalyst comprising an iron compound and an alkaline metal, treating the resultant reaction mixture in the presence of an oxidation catalyst comprising a noble metal in Group 8 and tin to thereby selectively oxidize the hydrogen contained in the mixture, reheating the treated mixture to conduct dehydrogenation reaction again, and recovering the dehydrogenated hydrocarbon.
As a result of investigations made by the present inventors, the following has been found. In the oxidative dehydrogenation method involving the selective oxidation reaction of hydrogen, when the mixture of ethylbenzene, hydrogen, etc. to be fed to the oxidation catalyst contains alkaline substances, then the selectivity of the catalyst is impaired since the alkaline substances deposit on the catalyst. Because of this, the hydrocarbons including ethylbenzene burn on the oxidation catalyst, resulting in an increased amount of carbon dioxide yielded.
It is, for example, known that potassium compounds are contained in ethylbenzene dehydrogenation catalysts and that such potassium compounds fly off during the dehydrogenation reaction (B. D. Herzog et. al., Ind. Eng. Chem. Prod. Res. Dev. 23, (2), 187(1984); Hayasaka et. al., Dai 24-kai Nippon Hokozoku Kogyokai Taikai Yoshi-shu, p.36 (1990); etc.).
If those potassium compounds fly off in a process in which dehydrogenation and the selective oxidation of hydrogen are alternately conducted in series, the oxidation catalyst comes to have considerably impaired selectivity.
On the other hand, carbon dioxide is known to serve to reduce the dehydrogenation activity of dehydrogenation catalysts (Hirano, Shokubai, 29, (8), 641 (1987), etc.). Consequently, an increase in the amount of carbon dioxide yielded in the oxidation step is undesirable for the subsequent dehydrogenation reaction because it means a suppression in conversion in the downstream dehydrogenation reaction.
An object of the present invention is to provide a method for preventing the hydrogen oxidation selectivity of an oxidation catalyst from lowering in a process which comprises conducting the dehydrogenation reaction of ethylbenzene to yield a reaction mixture containing styrene and hydrogen, burning the hydrogen by selective oxidation reaction, and further subjecting the unreacted ethylbenzene contained in the mixture to dehydrogenation reaction to produce styrene.
The present inventors made intensive investigations in order to eliminate the above problem. As a result, they have found that the oxidation reaction of hydrogen can be caused to proceed without impairing the selectivity of a catalyst for selective hydrogen oxidation, by removing alkaline substances contained in a slight amount in a dehydrogenation reaction product when the reaction product is located downstream from a layer of an ethylbenzene dehydrogenation reaction catalyst and upstream from a layer of the catalyst for the selective oxidation of hydrogen contained in the dehydrogenation reaction product. The present invention has been completed based on this finding.
The essential point of the present invention resides in a process for producing styrene by the dehydrogenation reaction of ethylbenzene which comprises at least the following steps (1) to (3):
step (1): a step of dehydrogenating ethylbenzene in the presence of a dehydrogenation catalyst to obtain a reaction mixture containing styrene and hydrogen;
step (2): a step of bringing the reaction mixture into contact with an oxidation catalyst to selectively oxidize the hydrogen contained in the mixture into water;
step (3): a step of bringing the oxidized mixture into contact with a dehydrogenation catalyst to dehydrogenate the unreacted ethylbenzene contained in the mixture, thereby to obtain styrene,
characterized in that an alkaline substance contained in the reaction mixture to be fed to step (2) is removed from the mixture beforehand.